This invention relates to an improved extruder screw design for use in the plastics industry where solid feed materials are transformed into a melted product by the application of heat and compression. This transformation takes place while the feed mix is being conveyed through the extruding machine.
As is known in the molding art, an extruder is a machine that has a helical screw that rotates within a fixed housing, jacketed for heating/cooling. The solid mixture being processed is fed into the machine at one end through a hopper arrangement, and the melted product is discharged or extruded out the other end.
The typical extruder screw, as described and illustrated in the following U.S. Pat. Nos. 4,310,484 (Blakeslee, III) and 4,129,386 (Rauwendaal), consists of at least three (3) defined sections. The section where the solid feed mix is introduced into the housing through a hoppered entry opening is termed the feed section. Next is a transition section, sometimes referred to as compression section, where some compression of the mix, along with melting thereof, takes place. The third or typically the final section in sequence is termed the metering section. In this area of the extruder, the partially melted mix and the remaining solid feed are conveyed toward the discharge opening. The final melting of the mix takes place in this last section. While the conventional extruder screw contains such three (3) sections, some more complex designs may call for a second feed section, for example, and corresponding transition and metering sections.
There are two important opposing flows within an extruder in operation. The forward flow due to the action of the helical screw forcing the material forward and through the heated/cooled housing is termed the drag flow component. Acting in opposition to the drag flow component is the pressure flow component. It is the result of the force developed from the total resistances encountered as the helical screw forces the molding material through the extruder housing. According to Miller et. al., U.S. Pat. No. 4,357,291, the speed of the screw does not directly affect the pressure flow component. However, it may affect such factors as back pressure and material viscosity, which factors, in turn, affect significantly the pressure flow component. On the otherhand, pressure flow is directly affected by both the depth and length of the screw channel. Thus, an increase in channel depth has a tendency to increase greatly the pressure flow component, while an increase in channel length has a tendency to reduce this back flow component.
Another complicating factor in the design of extruder screws is the molding material itself. Many formulations encountered in the extrusion of plastic molding compounds have components with a wide range of physical characteristics including particle size and density. This fact makes it difficult to maintain a homogeneous mix through the extrusion process, resulting in variations in product density. Due to inefficient conveying characteristics, i.e. drag flow, of existing designs, vascillations of flow will occur when pressure flow momentarily will exceed the drag flow, causing a back flow of material. This phenomenon of back flow is evidenced by accentuated wear on the screw and undercutting of the flighting, where this takes place. This wear is concentrated near the base of the flighting at its juncture with the root diameter. The wear is especially pronounced on screws processing formulations containing high proportions of abrasive materials.
From the above, as well as other prior art, there clearly has been a recognition of the forces acting within an extruder apparatus. Further, means have been proposed to try to control or contain such forces in order to optimize efficiency of operation and quality of product. For example, in Blakeslee, III, U.S. Pat. No. 4,310,484 an extruder screw having a decreasing pitch, the distance from a point on one flight to a corresponding point on an adjacent flight, from the feed section to the metering section has been proposed as a way of reducing the problem of back flow. Additionally, the patentee proposed the use of such decreased pitch over an extended length of flighting.
The extent of the available prior art would suggest that no single authority exists on the resolution or control of the forces within the extruder. With the present invention, an extruder screw has been designed which will allow the judicious increase in drag flow forces, and corresponding decrease in the pressure flow forces, while producing with duplicity a homogeneous, quality product at increased capacities over existing screw designs. The features thereof will become apparent from the specification which follows.